If you're about to fly somewhere for Thanksgiving, you're probably dreading the possibility that you'll catch a bug along the way. And you're not alone. Many people report coming down with something in the days following an airplane flight. It's not like we're all lining up to lick the lavatory handles — so why can't our immune systems handle a plane trip? What are our bodies really up against?

Two people I met on a recent flight prompted me to find out. I met Tom and Nancy back in September on a trip from Boston to Phoenix. The two of them were on their way to San Francisco to visit with Tom's sister for the week. A few seconds after we'd introduced ourselves, the smell of antiseptic drew my attention away from the baggage handlers on the tarmac loading luggage into the belly of the plane; each of my row-mates was armed with a disinfectant wipe. Tom was busy wiping down every visible face of his tray table, while Nancy had already moved on to dabbing purposefully at her seat's headrest. I asked her if she brings her cleaning supplies on all her plane flights.

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"Our doctor recommended it to us a few years ago," she said as she reached for her clutch bag, withdrew a powder-blue face mask and began looping its elastic straps over her ears. "We've been bringing wipes on all our plane flights ever since."

And the mask, I ask her. What is that for? "It's to help filter the air," she explained from behind the mask, its pleats expanding slightly as she mouths-out each word. "The air you're breathing is being re-circulated through the cabin — we're all breathing one another's air.

"It's disgusting," she concluded as she reached back into her bag and pulled out two more masks. She handed one to Tom and presents the other one to me. "Would you like one?"

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So, do we have to turn into Nancy and Tom to stay healthy on a plane flight? Yes and no.

What "recirculated air" really is

As it turns out, Nancy and Tom are right to be paranoid about the microscopic bugs that might be crawling around on their tray table (more on this later), but you might be surprised to learn that their concern over the cleanliness of the cabin's circulating air supply is largely unfounded.

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The idea of a germ-ridden, recycled, in-flight air supply is one of the most widely propagated urban myths about airline sanitation out there. As a result, many people (Nancy, Tom and — until recently — myself included) assume that if so much as one person on their flight is sick with something, everyone on board will have the misfortune of breathing in that person's germs — all thanks to the cabin's circulated air supply.

In reality, however, the air you breath on a typical airplane flight is remarkably sanitary. Fresh air from outside the plane is continuously drawn into the cabin via what are known as compressor stages in the jet's engines. These stages compress the very cold and extremely thin air from outside the plane until its pressure matches that of the cabin. Pressurizing the air also heats it up, so it's cooled back down before passing through High-Efficiency Particulate Air (HEPA) filters (which remove a minimum of 99.97% of any airborne particulates, bacteria and viruses) and combining with recirculated cabin air.

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But there's that word again. Recirculated.

Yes, the fresh air from outside the plane combines with some air that's already been making the rounds in the cabin for a little while — but that circulating air started out as fresh external air itself, and it too has been cycling through HEPA filters. What's more, recirculating cabin air is continuously released from the plane via outflow valves, so air inside the plane is constantly being replaced by the fresh air from outside. In fact, the average airplane's cabin air is completely refreshed about 20 times per hour. By comparison, the air in your average office building (which is also typically HEPA-filtered) is refreshed just 12 times per hour. In other words, the air you breathe at cruising altitude is most likely significantly cleaner than just about any you're liable to find on the ground.

Bugs, bugs everywhere

But let's get back to the wipes that Tom, Nancy and I used to disinfect our tray tables, arm rests and seat backs. Could these surfaces really pose that serious a risk of infection? Did disinfectant really need to be brought into the equation?

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To find out, I spoke with microbiologist Charles Gerba. Gerba heads up a research lab in the University of Arizona's College of Agriculture and Life Sciences, and is an expert on what are known in the medical community as "fomites" — inanimate objects or materials that are liable to carry and transmit infection. The term is commonly used to describe potentially dangerous surfaces in hospital settings, but much of the research in the Gerba lab tends to look at how diseases spread via surfaces found in a variety of other indoor environments, like public restrooms and airplanes.

Gerba confirms to me that the plane's air supply should be the last of my concerns when I take to the skies. He even tells me that on most airplanes, the air is circulated throughout the cabin not from front to back, but from top to bottom. This keeps air supplies localized throughout the plane, so even if you are "sharing somebody else's air" (which, remember, is being constantly filtered and refreshed, anyway) you're only sharing it with the people in your immediate vicinity — not the entire cabin.

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But we don't linger on the subject of ventilation for long. Our discussion quickly shifts to the real reason you're always getting sick on airplanes: having so many people in such a confined space means that just about every surface on your typical plane is packing some serious fomite potential.

"Where are you most likely to catch something on a plane?" Gerba repeats my question back to me. "Probably wherever you're sitting."

Avoid the aisle

Gerba can rattle off horror stories that will make you never want to fly on an airplane again. It's a good idea to avoid aisle seats, for example, because according to Gerba, those are the ones most likely to come in contact with — and therefore be contaminated by — other members of your flight. He offers up an extreme example to illustrate why this is. In 2008, members of a tour group afflicted with norovirus (a wicked stomach virus transmitted in fecal matter via food, people, and infected surfaces) came down with symptoms of uncontrollable vomiting and diarrhea during a flight from Boston to Los Angeles.

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According to Gerba, it was the very definition of a shit show. Infected passengers were experiencing diarrhea and vomiting throughout the plane (including one accident smack dab in the middle of first class) as they rushed to use the plane's lavatories and dispose of barf bags. Conditions got so awful the flight had to reroute for an emergency landing in Chicago, where infected passengers were rushed off to hospitals for treatment.

When the CDC contacted the plane's other passengers to follow up on whether or not they later came down with norovirus, they found that the ones most likely to have contracted the illness had been sitting in an isle seat. Obviously because people in the aisle seats were closer to the infected passengers who were moving throughout the plane, said Gerba, but also because people tend to use seats to stable themselves as they walk about the cabin. Infections were likely passed along via contact made with surfaces touched by the plane's sick passengers. (The figure shown here illustrates the seating on the flight with transmission of norovirus among passengers.)

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Ground Zero: The Toilets

This is obviously an extreme case, but Gerba says it helps underscore one of the reasons that airplanes (even more than subways, trains, and buses) — are especially prone to harboring illness. "Most people have probably never counted," says Gerba, "but your average plane flight will have just one toilet per fifty people; and [on some flights], that number is closer to 75."

"And remember," Gerba reminds me, "these planes see hundreds of passengers a day." When you combine a lot of people with not a lot of restrooms, he explains, you start seeing much higher rates of bacteria and viruses. That includes everything from norovirus, to seasonal flu, to the common cold, to Escheria coli.

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These bugs obviously appear in especially high numbers in the plane's restrooms. "You find a lot of E. coli on lavatory surfaces — even more than your typical public bathroom" says Gerba. "Especially around those tiny sinks, because passengers with big hands can't fit their hands in." Which is a real shame, I chime in, because washing your hands is obviously important. But here's the rub: Gerba tells me that even if you can fit your hands in the sink, it's probably best to bring some hand sanitizer along to be extra sure you're killing off as many microorganisms as possible. Why? Because even the sink water could be contaminated. "The EPA recently got on airlines for having choliform bacteria (fecal bacteria) in the airplane's water supply" Gerba says. "They've done a number of studies, and have come down pretty hard."

Fortunately, the EPA's efforts have translated to improved water quality. Things are still far from perfect (from 2005 to 2008, 3.6 percent of random airplane water samples still tested positive for fecal bacteria), but at least the EPA's Aircraft Drinking Water Rule, promulgated in October of 2009, helps keep contaminated water out of, for example, the drinks of passengers.

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Beware of your chair and tray table

But I've always been diligent about hand-washing on planes. I've even brought hand sanitizer with me in the past — so why am I still getting sick?

According to Jonathan Sexton, a researcher in Gerba's lab, infectious bugs aren't restricted to an airplane's restrooms.

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Your tray table, your complimentary pillow (which is often recycled flight after flight), the arms of your seat, your seat back pocket, the contents of your seat back pocket — these are all potential fomites, and evidence suggests that, like the lavatories, they're more contaminated than the surfaces in your typical public environment.

Consider, for example, a study conducted by Sexton back in 2007. Sexton collected samples from a variety of surfaces across numerous everyday environments (including airplanes) and analyzed them for Methicillin-resistant Staphylococcus Aureus (aka MRSA — a deadly superbug that before the 1990's was found primarily in hospitals, but has since begun making its presence known in the community).

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"We went ahead and tested multiple tray tables across three different planes. Sixty percent of [the tray tables, across all of the planes] tested positive for MRSA.

"We [also] found it in personal vehicles, offices, workplaces — everywhere. In just about every instance it appeared more often in the airplanes, but that could also be due to our smaller sample size [of three airplanes]."

By comparison, Sexton found MRSA in 3% of personal vehicles, 3.24% of work offices, 6.25% of public restrooms, and about a third of the home offices tested — opposed to one hundred percent of the inspected planes.

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"I always make sure that if I use my tray table — which I don't do very often — I've wiped it off with a clorox wipe and carry hand sanitizer," says Sexton. He continues:

You also always want to make sure if you have any open wounds that you have them covered up. Always make sure you wash your hands before you eat, or if you're coming into contact with any "high-touch" areas. It's good to be mindful of that.

Here's the takeaway: How to stay healthy on your next flight

The next time you're packing your carry-on for a flight, side with the disinfectant wipes and hand sanitizer over the filter mask; at the end of the day, it's coming into contact with a plane's countless potentially infected surfaces that's going to do you in — not the cabin's air supply (though the face mask could certainly help remind you to keep your fingers away from your mouth and nose, so go ahead and bring that along with you if you feel like making those around you feel especially unclean).

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When you combine the risk of infection-exposure with the stress of travel, dehydration (airplanes are notoriously dry), and the body-wrecking effects of jet lag, it's no wonder people often fall ill after a plane flight. So remember to be vigilant about the surfaces you come into contact with; bring some clorox wipes and some hand sanitizer on your next trip; and make sure your water comes from a bottle — or at least somewhere other than the plane.